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Functions
2.7 Thermal Overload Protection 49
Example::
Belted cable 10 kV 150 mm
Permissible continuous current Ι
Current transformers 400 A/5 A
[formel-therm-ueberl-k-fakt-2-oz-020802, 1, en_US]
Setting value 49 K-FACTOR = 0.80
Time Constant
The thermal time constant τ
turer. Please note that the time constant is set in minutes. Quite often other values for determining the time
constant are stated which can be converted into the time constant as follows:
1-s current
[fo_perm_1.0-s-continuous-current, 1, en_US]
permissible current for application time other than 1 s, e.g. for 0.5 s
[fo_perm_0.5-s-continuous-current, 1, en_US]
t
time; this is the time in seconds for which a current of 6 times the rated current of the protected object may
6
flow
[formel-therm-ueberl-zeitkonst-3-oz-020802, 1, en_US]
Example::
Cable as above with
Permissible 1-s current 13.5 kA
[formel-therm-ueberl-zeitkonst-4-oz-020802, 1, en_US]
Setting value TIME CONSTANT = 29,4 min
Warning Temperature Level
By setting a thermal alarm stage 49 Θ ALARM (address 4204) an alarm can be provided before the tripping
temperature is reached, so that a trip can be avoided by preventive load reduction or by switching over. The
percentage is referred to the tripping temperature rise.
The current overload alarm setpoint I ALARM (address 4205) is stated as a factor of the nominal device
current and should be set equal to or slightly below the permissible continuous current k · IN. It can also be
used instead of the thermal alarm stage. In this case the thermal alarm stage is set to 100 % and thus practi-
cally ineffective.
106
2
= 322 A
max
is set at address 4203 TIME CONSTANT. This is also provided by the manufac-
th
SIPROTEC 4, 7SD80, Manual
E50417-G1100-C474-A2, Edition 02.2018
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